During the preparation of biochar, the main preparation methods can be divided into pyrolysis and hydrothermal carbonization, depending on the pyrolysis technology used. Several key factors influence the biochar preparation process, including carbonization temperature, carbonization rate, carbonization pressure, reaction residence time, catalysts used, gas residence time, and the type of biomass. These factors collectively determine the properties and yield of the final biochar.
Pyrolysis:
Slow pyrolysis: Conducted at lower temperatures (300-650°C) and longer reaction times (usually less than 30 minutes), mainly producing gases and solid biochar. Slow pyrolysis is characterized by a high biochar yield, which can reach 35%.
Fast pyrolysis: Conducted at relatively high temperatures and very fast heating rates, with a very short product residence time, mainly producing bio-oil and a small amount of biochar, with a biochar yield of approximately 12% or less.
Microwave pyrolysis: Uses microwave energy to directly heat the biomass, resulting in a fast heating rate and simple operation. Although microwave pyrolysis has a low biochar yield, it offers more efficient energy utilization and faster reaction speeds.
Hydrothermal Carbonization:
This method involves reacting biomass in subcritical or supercritical water at 200-300°C in a sealed system. It does not require pre-drying of the raw materials and is suitable for processing biomass with high moisture content. Hydrothermal carbonization products have a high carbon content, and the process is controllable, making it suitable for preparing uniform carbon materials.
Gasification:
Biomass is gasified at high temperatures in a gasifier to produce combustible gases and biochar. Depending on whether a gasifying agent is used, it can be divided into gasification with a gasifying agent (such as air, steam, oxygen, etc.) and gasification without a gasifying agent (dry distillation gasification). This method can significantly reduce the content of other elements in the biochar.
Activation Methods:
Acid activation: Biochar is treated with acids (such as phosphoric acid, sulfuric acid) to improve its physical properties and adsorption capacity, and enhance its chemical activity.
Alkaline activation: Biomass or biochar is treated in an alkaline solution to increase the specific surface area and pore density, and enhance the adsorption capacity of negatively charged substances. Metal impregnation activation: Introducing metals or metal salts before or after pyrolysis to endow biochar with specific catalytic and adsorption properties.
Gas activation: Treating biochar with gases (such as steam, CO₂, ozone, etc.) at high temperatures to increase its surface area and pore structure.
